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In article <414@gtx.com>, al@gtx.com (0732) writes:
> One often hears about people blowing whistles or air horns into a
> telephone to thwart obscene callers. Is a telephone capable of
> transmitting enough sound amplitude to cause pain or damage to the ears?
> If so, this seems to be a dangerous capability in the hands of some crank.

	The general answer to your question is that acoustic injury via
telephone is improbable, but not impossible, depending upon specific
circumstances which I will outline below.  Bear in mind that there is
a substantial difference in sound energy required to make a listener
"uncomfortable" versus the energy level required to say, damage the
tympanic membrane.
	First of all, consider the receiver element in a telephone handset.
Almost all receiver elements in use today are electrodynamic transducers,
not unlike that of a radio speaker.  In a typical receiver element, the
changing voltage as applied to an electromagnet coil creates a variable
magnetic field which causes a magnetized, thin metal diaphragm to flex,
thereby compressing and rarefying the air, thereby creating sound waves.
	Now the point is that this diaphragm can only move SO much,
regardless of the magnitude of applied voltage, thereby _limiting_ the
maximum sound level it can produce.  A typical electrodynamic receiver
element, such as the "U1", will "top out" at well under 120 dB SPL,
even at its peak reproduction frequency (say, around 1 kHz).  For what
might be referred to as "casual" exposure, 120 db SPL is not going to
cause any physiological injury.  As a point of comparison, the threshhold
of pain is generally considered to be 140 dB SPL.
	Second, there are limits to the amount of electrical energy which
can be transmitted over a dialed telephone connection.  Audio frequency
energy transmitted over a telephone line will not be propagated beyond a
certain level; the reason is that certain telephone network components
such as transformers and inductors will magnetically saturate and thereby
limit transmission level.  Furthermore, active functional components of
the telephone network such as amplifiers, repeaters, PCM codecs, etc.
will also saturate and not pass audio energy beyond a certain level.
	In general terms, if one were to electrically connect an audio
frequency generator to a dialed telephone connection, regardless of the
magntiude of input energy, it is improbable that more than 20 dBm (100 mW)
of energy could be delivered to the receiving telephone IF the call
were switched through an electromechanical central office, and IF both
the calling and called party were in the SAME central office.  If the
telephone central office were ESS and/or if the called party was served
by a different central office, it is highly improbable that more than
10 dBm (10 mW) of energy could be delivered to the receiving telephone.
	Even using the worse-case figure of 100 mW as applied to say, a
U1 receiver, it is improbable that even 110 dB SPL could be achieved -
still a far cry below even the threshhold of pain.  Furthermore, at
this kind of energy level, the "click-suppression" varistor across the
U1 handset would begin some serious conduction, thereby shunting energy
away from the receiver.
	Third, a telephone transmitter - either traditional carbon or 
the newer reluctance variety - will saturate if a sound level (like a
boat horn) is applied to it, thereby limiting the amount of any electrical
signal it may generate.
	So, for any or all of the above reasons, it is improbable that
sufficient acoustic energy (sound pressure level in dB) could be created
such that physiological injury would occur.
	There is one factor that could contribute to possible physiological
injury - but it is not commonly found.  I will relate a little firsthand
story...
	As a combination EE/biochemist with a multidisciplinary background,
I have done some forensic science consulting for a number of years.  About
ten years ago, I was retained as a consultant in a workmen's compensation
case.  It seems that a middle-aged woman who had worked as a switchboard
operator for 20-some years had applied for compensation resulting from
partial hearing loss in one ear.  Her allegation was that then presence
of clicks and loud noises in her "headset ear" over many years caused her
hearing loss.  My function was to work with an otolaryngologist to disprove
her claim.
	My first reaction was that there was insufficient sound pressure
level to cause such injury.  However, I learned that for the past five
years the woman used a Plantronics miniature headset (the kind with a
flexible plastic ear tube).  Now with such a headset, there was an almost
perfect sound conduction between the receiver element and the ear.  So,
we contacted Plantronics and got some engineering information.  We got a
sample headset and conducted our own sound pressure measurements using
a Bruel & Kjaer "artificial ear".  The conclusion was that the maximum
sound pressure level which could be experienced through this headset
in a switchboard environment was still insufficent (by generally accepted
medical criteria) to cause hearing impairment.
	But the use of a receiver with a plastic ear tube did make me
seriously consider the possibility of injury.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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